The present invention relates generally to vacuum-operated sewerage control systems utilizing inlet vacuum valves, and more specifically to such a system employing an electric air admission controller ("EAAC") for opening the vacuum valve independently of the level of sewage accumulated in a holding tank at the inlet of the valve, in order to introduce additional air at atmospheric pressure into the sewerage transport conduit to avoid a waterlogged state.
Sewerage systems initially were gravity operated, including a network of underground pipes leading from various sources of waste (e.g., homes, businesses, etc.) to a sewage treatment plant. However, irregular terrains and distances posed between the entry and collection points of the waste significantly limited the ability to dig deep trenches to provide a continuous, downhill flow of sewage. Thus, mechanical pumps were placed at strategic points along the pipe network to provide a positive force behind waste flowing in a more-shallowly laid piping network. In actuality, though, pressure pumps were needed at every sewage input point for such a system.
Vacuum-operated systems were proposed as an alternative, as exemplified by U.S. Pat. No. 3,115,118, issued to S. A. J. Liljendahl. The Liljendahl patent describes a vacuum system which uses two separate piping networks to carry different effluent streams. While the waste products from bathtubs, wash basins, sinks, etc. (gray water), are conveyed by a conventional gravity system, waste products discharged from water closet bowls, urinals, and similar sanitary apparati (black water) are transported by a separate, vacuum system. The conduits in the latter system are provided with "pockets" in which sewage is collected so as to form a plug which entirely fills the cross-sectional area of the pipe and effects conduit closure. A plug of sewage is moved by a pressure differential force along a conduit in an integral condition. The kind of vacuum-operated system taught by the Liljendahl patent is called "plug flow."
U.S. Pat. No. 3,730,884, issued to B. C. Burns et al., describes a sewerage system using "vacuum-induced plug flow" in which both black water and gray water are handled by a single piping system. A "coherent plug" of sewage is transported by a vacuum pressure differential through a pipe for a short distance. The plug will disintegrate, however, as it moves through the pipe due to friction and other forces, resulting in a diminishment of the pressure differential moving the plug. Therefore, a series of plug reformers, which in their simplest form may be a dip or pocket in the pipe, serve as a trap for sewage and aid in the reformation of a coherent plug. The pockets are designed so that sewage entirely fills the pipe bore. Operation of the system requires that the plug of sewage seal the pipe bore. This process of alternate plug disintegration and reformation continues until the sewage eventually passes completely through the pipe. The pressure differential for each of these plugs is less than the total available system pressure differential because of the serial arrangement of the plug pockets in a pipe.
U.S. Pat. No. 4,179,371, issued to B. E. Foreman et al., describes an apparatus and method for transporting sewage from a source of sewage to a collection means. A pressure differential is maintained in the piping between the source and the collection means. Sewage is transported through the conduit in the form of a generally hollow cylinder. When no sewage is being transported, the residual sewage retained in the conduit generally does not close the conduit, and permits the same pressure to be maintained throughout the conduit. Injection means are provided, which may be a valve intermittently opened in response to a predetermined condition. The conduit is laid out in a saw-toothed configuration with a combination of a riser portion, a downslope portion, and a low point portion (collectively called a "lift") in which residual sewage not discharged from the system may collect. The residual sewage is generally insufficient to close the conduit, thereby permitting communication of the same pressure throughout the conduit. Thus, the apparatus, as disclosed by Foreman, may include a gravity-fed sewage collection tank at atmospheric pressure having its contents intermittently injected into a vacuum-pressurized conduit laid out in saw-toothed fashion, which permits full vacuum to be communicated throughout the conduit under typical operating conditions.
A problem encountered by vacuum transport sewerage systems is "waterlogging." As already discussed, the collection of residual waste material in the low point portions of the conduit, under normal operating conditions, is insufficient to seal the conduit at low points, and is designed to maintain throughout the conduit an air space in the conduit to permit pressure communication. During a sewage transport cycle, the total conduit volume will typically be less than one-third liquid. However, if an insufficient amount of atmospheric air is introduced into the conduit, there will be insufficient energy applied to move effectively the entire waste mass during the sewage transport cycle. This leads to an increased accumulation of residual waste material, thereby creating the waterlogged condition which might fill two-thirds of the conduit and lift volumes.
There are a number of potential causes of waterlogging. For example, the valve assembly may be misadjusted so that the valve closes too quickly after the waste material has entered the conduit, thereby undesirably reducing the amount of atmospheric air entering the conduit and pulling the waste material along. Likewise, leakage somewhere in the system will impair the maintenance of vacuum or subatmospheric pressure in the conduit so that over time, this vacuum or subatmospheric pressure condition will decrease to the point that the differential pressure during transport cycles will be insufficient to move waste product, resulting in waterlogging. Also, while the conduit network will equalize to a vacuum or subatmospheric pressure after the valve closes, terminating the sewage transport cycle, it will do so at a slightly lower vacuum pressure due to inefficiency in the system or improper operation of the source of vacuum or subatmospheric pressure. This, in turn, contributes to a deficient level of vacuum or subatmospheric pressure and, therefore, pressure differential.
While waterlogging theoretically may occur no matter how many feet of lift are in the conduit line, the probability will increase, as the "total lift" is increased, because it will be more difficult for gravity fall and the vacuum to lift sewage within the conduit lines at each succeeding profile change. Assuming that each saw-toothed lift consists of a lower downslope portion, a riser, and an upper downslope portion, the pertinent distance is measured vertically between the point on the bottom exterior surface of the upper downslope portion where it joins the riser, and the point on the top exterior surface of the lower downslope portion where it joins the same riser. Aggregating these distances across the lifts of the flow path produces a measurement for the "total lift."
Typically, a vacuum system operates within a range of 16" Hg to 20" Hg vacuum. Because atmospheric pressure, on the other hand, is defined as 0" Hg in this vacuum pressure scale, this represents "a pressure differential" of 16" Hg to 20" Hg also. Taking the minimum available vacuum level of 16" Hg, and subtracting 5" Hg which must be present at all times to operate the vacuum valves and their controls, leaves 11" Hg vacuum available for vacuum lift in the mains. Eleven inches of mercury is equivalent to 12.5 feet of water, which is typically rounded up to 13 feet. Thus, 13 feet of lift is typically the maximum figure used in the design of the vacuum mains for any sewer project.
Total lift of approximately 13 feet is important for two reasons. First, any system with less than 13 feet of lift which waterlogs can theoretically correct itself over time through normal valve cycling to purge the accumulated residual sewage from the conduit line. By contrast, a waterlogged vacuum transport system designed with 13 feet or more of total lift traditionally has needed operator assistance to purge the residual sewage. Because the valve opens in response to differential pressure based upon the vacuum pressure condition in the conduit immediately downstream of the valve, if that vacuum pressure is too low due to waterlogging blockage of the conduit, the valve will not cycle to introduce atmospheric pressure into the conduit, thereby preventing the conduit from automatically unwaterlogging itself over time. Instead, the repairman will have to restore the source of vacuum pressure in the system, move upstream to a valve having adequate vacuum pressure able to be activated and activate that valve, and then progressively activate each valve further upstream until the vacuum mains are cleared of the waterlogged sewage.
The second important aspect of the 13-foot measurement is that it presents a limitation on the overall length of the sewerage transport lines. Combination of the predetermined slope of the lines with a maximum total lift of 13 feet determines the maximum distance the vacuum lines may travel to ensure proper sewage flow without the aid of mechanical pumps. Actually, the total permitted lift across the flow path is limited additionally by a frictional loss factor calculated according to various formulae known in the art of fluid dynamics.
In order to operate effectively a vacuum sewage system at loss levels exceeding about 13 feet, a higher air-to-liquid ratio is used. This may be simply accomplished by admitting more air into the conduit. Typical systems operating at or below the 13-foot level may be designed at a 3-to-1 air-to-liquid ratio. This number can be proportionately increased to increase lift within the system.